The present invention relates generally to methods for treating a subterranean formation with a solid acid chelating agent.
Subterranean formations from which oil and/or gas can be recovered can contain several solid materials contained in porous or fractured rock formations. The naturally occurring hydrocarbons, such as oil and/or gas, are trapped by the overlying rock formations with lower permeability. The reservoirs are found using hydrocarbon exploration methods and often one of the treatments needed to withdraw the oil and/or gas therefrom is to improve the permeability of the formations. The rock formations can be distinguished by their major components.
One process to make formations like carbonate or sandstone formations more permeable is an acid fracturing process, wherein an acidic fluid is introduced into the formations trapping the oil and/or gas under a pressure that is high enough to fracture the rock, the acidic fluid meanwhile or afterwards dissolving the carbonate so that the fracture does not fully close anymore once the pressure is released again. In carbonate formations, the goal is usually to have the acid dissolve the carbonate rock to form highly-conductive fluid flow channels, which are called wormholes, in the formation rock usually under flow injection regimes that are not conducive to the fracturing of the rock, also known as matrix acidizing.
In acidizing a carbonate, dolomite, or a combination thereof formation, calcium and magnesium carbonates of the rock can be dissolved with acid. A reaction between an acid and the minerals calcite (CaCO3) or dolomite (CaMg(CO3)2) can enhance the fluid flow properties of the rock.
Common acids such as hydrochloric acid (HCl), acetic acid, and formic acid are typically used in acidizing. These acids, however, can have adverse effects when certain downhole well conditions are encountered. Typical problems occur when the wells reach an elevated temperature, which leads to near well-bore (NWB) spending and increased corrosion.
NWB spending leads to the need for increased volumes of acid to achieve penetration into the formation. Moreover, as temperatures increase, the acids exhibit increased reactivity with the formation such that NWB spending or softening of the formation leads to wellbore or NWB collapse or other adverse failures.
Corrosion is also a major factor when elevated temperatures are encountered in downhole conditions. As temperatures increase, acids can be inhibited with large acid inhibitor concentration loadings, which can lead to formation damage or fluid instability. In many instances, such as at temperatures above 350° F., common acids cannot be inhibited. In other instances, highly sensitive metallurgical components and completions (such as low carbon steel, chrome-type steels, and molybdenum-containing alloys like coiled tubing) are employed that restrict the use of HCl acid fluids.
Another problem encountered with acid treatment is the formation of sludge. HCl, particularly when at high concentrations of about 15% and greater, can cause the development of sludge when the acid is placed in contact with certain types of crude oil. The sludge formation problem is exacerbated when the acid that is in contact with the crude oil also contains ferric ion.
Certain crude oils contained in subterranean formations produce sludge upon contact with aqueous acid solutions during the carrying out of acidizing treatments. The sludge formed is an asphalt-like material which precipitates in the formations and often plugs or clogs the enlarged flow channels formed therein. Interaction studies between sludging crude oils and acids have shown that precipitated solids or films are formed at the acid oil interface. The precipitates are mainly asphaltenes, resins, paraffins and other high-molecular weight hydrocarbons.
When sludges are produced in crude oil, the viscosity of the oil drastically increases. Due to this increase, the rheological characteristics of the fluid can exhibit negative effects by a dramatic decrease in formation fluid-drainage properties. The treated formations are very slow to clean up, if at all, and often the acidizing treatments produce a decrease in permeability and reduction in oil production instead of an increase.
Another common cause of production declining in a mature hydrocarbon well is fouling of the perforations in the well casing and the structure of the formation around the well with scale precipitated from brine. These precipitations are known to form near the wellbore, inside casing, tubing, pipes, pumps and valves, and around heating coils. Reduction of near wellbore permeability, perforation tunnel diameter, production tubing diameter, and propped fracture conductivities can significantly reduce well productivity. Over time, large scale deposits can reduce fluid flow and heat transfer as well as promote corrosion and bacterial growth. As the deposits grow, the production rate decreases and even the whole operation could be forced to halt.
The production may be revived, at least partially, with a stimulation technique. One commonly used technique is hydraulic fracturing. In the process of hydraulic fracturing, a fracturing fluid is injected at high pressure into a subterranean formation to create artificial cracks in the subterranean formation. A proppant added to the fracturing fluid fills the fractures to maintain the openings created by the crack. Although the fracture exposes new rock and breaks scale, once the fracture has been made and hydrocarbon production resumed, the well and the adjacent subterranean formation are still subject to scaling from precipitating minerals from subterranean brines, for example, calcium sulfate and calcium carbonate.
Removal of scales often requires expensive well interventions involving bullhead or coil tubing placement of scale dissolving chemical treatments, milling operations or re-perforation. Economically efficient scale management predominantly involves the application of chemical scale inhibitors that prevent scale deposition. Scale inhibitors are conventionally applied as downhole injections or squeeze treatments. Since hydraulic fracturing is costly, sometimes costing as much as drilling the well in the first place, it is necessary that future build-up of scale be prevented as much as possible.
Thus, there is a continuing need for improved methods and compositions for treating subterranean formations. Specifically, there is a need for improved methods and compositions for acidizing in oil and gas operations. In particular, there is a need to control how fast the acid reacts and where in the formation the acid reacts. In addition, there is a need for reducing the formation of sludge in oil and gas operations and inhibiting the formation of scale in subterranean formations.